Comparative studies shed light on alternate neuroendocrine mechanisms underlying reproduction and its attendant sexual behaviors. By exploring alternate mechanisms and comparing them to the established paradigm, one can illuminate which neural substrates are basic and critical tot he expression of these behaviors. Bats are the second most prolific radiation of mammals yet they remain relatively unstudied. They display great diversity in their reproductive patterns and vespertilionid bats, in particular, constitute an excellent alternative model system to study neuroendocrine mechanisms affecting reproductive behavior due to unique features of their life history. They 1) have a dissociated pattern of reproduction, mating when gonads are regressed and sex steroids are basal, and 2) are heterothermic, with arousal from hibernation being a potent and rapid stimulator of sexual behavior. Therefore, an external cue (i.e. temperature change) appears to activate sexual behavior independent of the classical role played by sex steroids. Preliminary data indicate that male and female big brown bats, Eptesicus fuscus, a vespertilionid, can mate months (at least 6 for males and 18 for females) after gonadectomy when given the proper temperature regimen. The specific research objectives of this proposal are to test the hypotheses that this species has 1) become relatively (if not completely) independent of sex steroids as an activator of reproductive behavior and sexual behavior. If these hypotheses are supported, the PI proposes a starting point to explore which alternative neuroendocrine mechanisms may be replacing the classic mechanism of sex steroid activation of sexual behavior. To achieve these objectives, two intertwined lines of research will be established. One will document the extent to which the expression of sexual behavior by males and females is independent from gonadal sex steroids by removal of gonads at different points in the seasonal cycle as well as removal of possible extragonadal sources of sex steroids. Replacement sex steroids will be given at these points to determine their efficacy in re-instating the behavior in animals. The second line will document the pattern of torpor and arousal and how sex steroids change in relation to these states. It will also determine what degree of exposure to low temperature (or entrance into a torpid state) insures the expression of mating and how this is modified by the presence or absence of sex steroids. Preliminary experiments will administer intracerebroventricular doses of GnRH antagonist and agonist to begin testing if this is a potential neuroendocrine mechanism linking arousal from hibernation and mating in this species.